1. Field of the Invention
The present invention relates to a molded article for a negative electrode, a method of producing the same and a lithium secondary battery using such a molded article. More particularly, the present invention relates to a molded article useful for the negative electrode of a lithium secondary battery, a method of producing the same, and the lithium secondary battery using such a molded article as the negative electrode. The molded article for a negative electrode of the present invention is very useful for the negative electrode of a lithium secondary battery because the molded article has a favorable mechanical strength, e.g., a three-point bending strength of 100 kgf/cm.sup.2 or greater at room temperature, and sufficient energy density per unit volume to provide the lithium secondary battery with advantageous charge-discharge characteristics for higher loads.
2. Description of the Related Art
A lithium primary battery employing lithium metal as a negative electrode active substance has been widely used because it has advantages such as higher energy density, light weight, a compact size and long-term shelf life.
Though this lithium metal is effective when used as a negative electrode active substance in a lithium primary battery, if it is used as a negative electrode active substance in a lithium secondary battery, the use of lithium metal causes many problems which are not observed when it is used in a lithium primary battery. Because of such problems, it is not practical to use lithium metal as the negative electrode in a lithium secondary battery. More specifically, a lithium secondary battery having a lithium metal negative electrode has significant drawbacks such as short charge-discharge cycle life and low charge-discharge efficiency. These drawbacks result from deterioration of the negative electrode caused by an electrochemical reaction in which lithium metal is deposited in a dendritic form on the negative electrode during the processes of charge and discharge. A lithium secondary battery whose negative electrode is made of lithium metal inevitably experiences such deterioration of the negative electrode.
To solve the above problem which is the deterioration of the negative electrode when lithium metal is used in a secondary battery, many proposals have been made that negative electrodes be formed of carbon materials, such as pyrolysis residues of a conjugated polymer (Japanese Patent Application Laid-Open No. 58-093176), thermal decomposition products of polyarylacetylene (Japanese Patent Application Laid-Open No. 59-154763), pyrolysis residues of an organic compound (Japanese Patent Application Laid-Open Nos. 60-235372 and 62-090863), a carbon material having pseudo-graphite structure (Japanese Patent Application Laid-Open No. 62-122066) or a carbon molded article having a plane network six-membered ring structure (Japanese Patent Application Laid-Open No. 63-013282).
The lithium secondary batteries whose negative electrodes are formed of carbon materials utilize a reversible absorption-desorption reaction, i.e., an electrochemical reaction in which the carbon material absorbs lithium, i.e., a negative electrode active substance, during charge and releases the lithium into the electrolyte solution during discharge. Therefore, carbon particles forming the negative electrode, i.e., the carbon molded article (material), expand and shrink during charge and during discharge, i.e., while lithium is being absorbed and while it is being released, respectively.
Carbon materials, such as pyrolysis residues of polymers, used for the negative electrodes of lithium secondary batteries are generally in the form of powder. Therefore, to form negative electrodes of such carbon materials, the carbon materials must be molded in predetermined shapes by suitable methods such as pressure molding In general, a carbon material is homogeneously mixed with a polymer compound, e.g., polytetrafluoroethylene, polyethylene or polypropylene, as a binder, and then, the mixture is molded by pressure molding or roller molding. Another method in which a mixture of a solvent and a carbon material and a binder both dispersed in the solvent is applied to a current collector (an electrode member) is also known.
However, because the electrode (the positive and negative electrodes) formed by the above-described methods contains polymer compounds such as polytetrafluoroethylene or polypropylene added as a binder and, sometimes, a trace of the solvent, the electrodes have problems in that electric conductivity, which is supposed to be high due to the carbon materials, is reduced. More specifically, a battery having electrodes formed of a carbon material containing a polymer compound experiences increased overvoltage, which results in a reduced battery capacity and, in addition, heat generation. It has not been expected that a technique to solve these problems would be achieved.
As described above, in principle, a battery having electrodes formed as carbon molded articles containing a polymer compound, e.g, polytetrafluoroethylene and polypropylene, as a binder has significant problems in that overvoltage is increased so that the battery capacity is reduced and heat is generated
When a carbon molded article containing such a polymer compound as a binder is employed as the negative electrode in a lithium secondary battery, carbon particles forming the carbon molded article (the negative electrode) experience cycles of expansion and shrinkage, respectively, caused during charge and discharge, i.e., during absorption and release of lithium. Because of stress fatigue caused by the repeated cycles of expansion and shrinkage, the binding capacity of the polymer compound added as a binder decreases with time and finally results in breakage of the carbon molded article. Detached carbon particles from this article enter into the electrolyte solution and, in the worst case, cause a short between the positive and negative electrodes. Thus, a carbon molded article for a negative electrode containing a polymer compound as a binder has a significant technical problem in that there is a potential danger of a short occurring between the electrodes.
The above problems connected with the use of a carbon molded article for a negative electrode must be solved in order that a lithium secondary battery having a negative electrode of a carbon molded article and advantageous charge-discharge characteristics for higher loads can be put into practical use.